(a) Field of the Invention
The invention relates to a voltage regulation device, particularly to a switching regulator.
(b) Description of the Related Art
Typically a switching regulator is a control circuit for maintaining an output voltage of the switching regulator a setting value, where the setting value is a reference voltage Vref of a circuit. When the circuit is powered on, the switching regulator provides the reference voltage Vref of a voltage level slowly increasing to a required voltage value. If the reference voltage Vref of a constant voltage level is immediately fed into the circuit when the circuit is powered on, a considerable sink current will occur at both the input and the output parts of the circuit. Though the sink current may not permanently damage the circuit or an off-chip device, it may affect their characteristics such as stability or duration. Therefore, to slowly increase the reference voltage is very important for the switching regulator when the system is powered on.
FIG. 1 shows the circuit structure of a typical switching regulator. Referring to FIG. 1, the switching regulator 100 is used to generate an output voltage Vout identical to the reference voltage Vref. The switching regulator 100 includes an EA (error amplifier) 102, a pulse-width modulator 104, a gate driver 106, a switching circuit 108, an inductor 110, and a capacitor 112. The error amplifier 102 receives the reference voltage Vref as well as the output voltage Vout and generates an error voltage signal. The pulse-width modulator 104 receives the error voltage signal as well as a periodic signal (such as a sawtooth clock) and generates a pulse-width modulation signal (such as a square clock). The gate driver 106 receives the pulse-width modulation signal of the pulse-width modulator 104 and generates a switch control signal. The switching circuit 108 receives the switch control signal and output a current signal. The switching circuit 108 includes two transistor switches SW1 and SW2, and their gates are controlled by the switch control signal. The inductor 110 and the capacitor 112 receive the current signal and generate the output voltage Vout.
The switching regulator compares the output voltage Vout with the reference voltage Vref to control the switches SW1 and SW2 of the switching circuit 108, so that a steady output voltage Vout is supplied. When the circuit is powered on, the output voltage Vout is 0 in the beginning, and then the output voltage is gradually increased to a steady voltage level. The operation of a switching regulator is conventional thus the detail description is omitted. The further detail is described in section 1.3.1 of the book entitled “Switching Power supply design” (2nd, Abraham I. Pressman).
When the switching regulator is powered on and the reference voltage Vref equals a preset output voltage, the considerable voltage difference will make the switch SW1 keep conducting to result in an abnormal enormous current flowing through switch devices and the system output. To avoid this problem, when the switching regulator is powered on, the reference voltage Vref must be increased slowly.
Generally, to slowly increase the reference voltage Vref is referred to as “soft start”, where a soft start circuit is utilized to generate the reference voltage Vref with gradual increases. FIG. 2 shows a schematic diagram illustrating a conventional soft-start circuit 200 in connection with the switching regulator 100. Referring to FIG. 2, the reference voltage Vref is increased gradually since the capacitor C1 outside the chip is charged. However, the method requires an additional pin, a large capacitor C1 outside the chip, and off-chip resistors R1 and R2.
When the soft start circuit is powered on, a constant current Ich is utilized to charge the off-chip capacitor C1 so that the reference voltage Vref is increased gradually at the pin SlowST and the pin Vrefb. When the voltage of the off-chip capacitor C1 reaches a preset value, the reference voltage Vref stays at a fixed voltage level generated by the constant current Ich and the off-chip resistors R1 and R2. However, after the reference voltage Vref reaches the preset value, to maintain the reference voltage Vref, an additional switch is needed to switch the original voltage source to another steady voltage source because the off-chip capacitor fails to be accurately controlled. Furthermore, the constant current Ich is adjustable to maintain the reference voltage accurately. Also, it is unfavorable to incorporate the capacitor C1 into the chip due to its off-chip capacitance.